On this page (9)
How to use this calculator#
- Measure your height without shoesStand with heels together against a wall, looking straight ahead, and measure to the top of your head. Height is squared in the formula, so a 2 cm error moves a BMI of 25 by roughly 0.6 — twice the effect of a 1 kg weighing error.
- Weigh yourself under consistent conditionsFirst thing in the morning, after using the bathroom, before eating or drinking, in light clothing. Body weight swings 1–2 kg through a normal day from food, fluid and glycogen, and that is enough to move BMI by half a point.
- Enter both figures in the same unit systemSwitch the toggle to metric or imperial and stay there. Mixing a metric weight with an imperial height is the single most common reason a BMI result looks wildly wrong.
- Read the band, not the decimalBMI 24.8 and BMI 25.2 are not meaningfully different bodies. Look at which band you sit in and how far from a boundary you are, and pair it with a waist measurement, which tracks abdominal fat that BMI cannot see.
The formula#
Quetelet index (Body Mass Index)
BMI = weight (kg) ÷ height (m)² • imperial: BMI = 703 × weight (lb) ÷ height (in)²
- weight (kg)
- Body mass in kilograms
- height (m)
- Standing height in metres — 175 cm is entered as 1.75
- 703
- Conversion constant that makes pounds and inches produce the same number as kg/m²
Devised by Adolphe Quetelet in the 1830s as a population statistic and only renamed 'body mass index' by Ancel Keys in 1972. It was never designed to assess an individual, which is exactly why every health authority describes it as a screening tool. The cut-offs used here are the WHO adult classification and apply to adults aged 20 and over only; children and teenagers are assessed against age- and sex-specific percentile charts instead.
What BMI tells you (and what it doesn't)#
BMI is weight (kg) divided by height (m) squared. It's a quick screening tool for weight categories across a population, but it doesn't distinguish muscle from fat or account for body composition.
Use BMI as a starting point, not a diagnosis. Athletes and very muscular people may read as ‘overweight’ despite low body fat.
Worked examples#
Metric calculation
An adult weighing 80 kg with a height of 180 cm.
- Convert height to metres: 180 cm ÷ 100 = 1.80 m
- Square it: 1.80 × 1.80 = 3.24
- Divide weight by that: 80 ÷ 3.24 = 24.69
BMI 24.7 — inside the healthy band of 18.5–24.9, but only 1 kg below the overweight boundary of 80.7 kg at this height.
Imperial calculation, and where BMI breaks down
A person at 5 ft 9 in and 165 lb, then the same height at 220 lb of largely muscular weight.
- Height in inches: (5 × 12) + 9 = 69 in, and 69² = 4,761
- BMI = 703 × 165 ÷ 4,761 = 115,995 ÷ 4,761 = 24.4
- At 220 lb: BMI = 703 × 220 ÷ 4,761 = 154,660 ÷ 4,761 = 32.5
24.4 (healthy) versus 32.5 (obesity class I). A heavily resistance-trained athlete at 220 lb and 12% body fat gets the same 32.5 as someone carrying the extra weight entirely as fat, because BMI has no way to tell the two apart. This is the misclassification every guideline warns about.
Reference tables#
| BMI (kg/m²) | Category | Notes |
|---|---|---|
| Below 16.0 | Severe thinness | Warrants clinical assessment |
| 16.0 – 18.4 | Underweight | Associated with nutrient shortfalls and low bone density |
| 18.5 – 24.9 | Healthy weight | Lowest population-level risk band |
| 25.0 – 29.9 | Overweight | Pre-obesity in WHO terminology |
| 30.0 – 34.9 | Obesity class I | Risk rises with waist circumference |
| 35.0 – 39.9 | Obesity class II | Sometimes described as severe obesity |
| 40.0 and above | Obesity class III | Sometimes described as very severe obesity |
These bands describe statistical risk across populations. They do not diagnose any individual, and an above-range BMI on its own is not evidence of disease.
| Height | Healthy range (kg) | Healthy range (lb) |
|---|---|---|
| 150 cm / 4 ft 11 in | 41.6 – 56.0 kg | 92 – 124 lb |
| 155 cm / 5 ft 1 in | 44.4 – 59.8 kg | 98 – 132 lb |
| 160 cm / 5 ft 3 in | 47.4 – 63.7 kg | 104 – 140 lb |
| 165 cm / 5 ft 5 in | 50.4 – 67.8 kg | 111 – 149 lb |
| 170 cm / 5 ft 7 in | 53.5 – 72.0 kg | 118 – 159 lb |
| 175 cm / 5 ft 9 in | 56.7 – 76.3 kg | 125 – 168 lb |
| 180 cm / 5 ft 11 in | 59.9 – 80.7 kg | 132 – 178 lb |
| 185 cm / 6 ft 1 in | 63.3 – 85.2 kg | 140 – 188 lb |
| 190 cm / 6 ft 3 in | 66.8 – 89.9 kg | 147 – 198 lb |
Each range is 18.5 × height² to 24.9 × height², with height in metres. The band widens with height because the square term grows faster than the weight.
| BMI (kg/m²) | WHO international | Asian public-health action point |
|---|---|---|
| 18.5 – 22.9 | Healthy weight | Increasing but acceptable risk |
| 23.0 – 24.9 | Healthy weight | Increased risk |
| 25.0 – 27.4 | Overweight | High risk |
| 27.5 and above | Overweight / obesity | Very high risk |
The consultation found that people of Asian descent tend to carry more body fat and higher cardiometabolic risk at a given BMI, and recommended 23.0 and 27.5 as additional public-health action points rather than replacing the international cut-offs. Several countries, including China, India, Japan and Singapore, use lower thresholds in their national guidance.
Common mistakes#
- Treating BMI as a body fat measurementBMI knows only your mass and your height. Muscle is denser than fat, so a rugby forward, a powerlifter or a sprinter routinely lands in the overweight or obese bands while carrying single-digit body fat. Pair BMI with a waist measurement or a body fat estimate before drawing any conclusion.
- Applying adult cut-offs to a child or teenagerChildren's body composition changes constantly with growth, so paediatric BMI is read against age- and sex-specific percentile charts, not the 18.5/25/30 bands. Using adult thresholds on a 12-year-old produces meaningless — and potentially harmful — labels.
- Ignoring that the cut-offs are population-specificThe WHO's 2004 expert consultation flagged that many Asian populations show elevated cardiometabolic risk from around BMI 23. Older adults also lose muscle and gain fat at a stable BMI, so the same number means something different at 75 than at 25.
- Chasing a decimal placeA 1 kg change or a 1 cm difference in measured height moves BMI by roughly 0.3. Reading significance into 24.9 versus 25.1 is reading noise. What matters is the direction of travel over months and what else your health picture shows.
Frequently asked questions#
What is a healthy BMI?
For most adults, a BMI of 18.5–24.9 is considered a healthy weight, 25–29.9 overweight, and 30+ obese. Under 18.5 is underweight.
Is BMI accurate for everyone?
It's a general screening tool. It can misclassify very muscular people, older adults and some ethnic groups, so treat it as one signal among many.
Key terms#
- Body Mass Index (BMI)
- Weight in kilograms divided by height in metres squared, expressed in kg/m². A screening index for weight relative to height across a population.
- Quetelet index
- The original name for BMI, after the Belgian statistician Adolphe Quetelet who described the weight-to-height-squared ratio in the 1830s as a way of characterising the 'average man'.
- Waist-to-height ratio
- Waist circumference divided by height. Keeping it below 0.5 is a widely used rule of thumb, and it captures abdominal fat — the distribution most linked with metabolic risk — which BMI cannot detect.
- Adiposity
- The amount and distribution of body fat. BMI is a proxy for adiposity, not a measure of it; DEXA, air displacement plethysmography and hydrostatic weighing measure it directly.
- BMI percentile
- How a child or adolescent's BMI compares with others of the same age and sex. Paediatric assessment uses percentiles because absolute BMI shifts predictably through growth.
- Obesity class
- The WHO's subdivision of BMI 30 and above into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0+), used to stratify risk and guide clinical pathways.
Sources#
- Obesity and overweight — fact sheet with the adult BMI classification — World Health Organization
- Appropriate body-mass index for Asian populations (WHO expert consultation, The Lancet 2004) — PubMed / WHO Expert Consultation
- Calculate your body mass index — NHS
- Overweight and obesity — definition and facts — NIH / NIDDK
Figures last checked .
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